This invention relates detecting RF link imbalances and more particularly to a method for determining RF link imbalances from the User Equipment side of the Air Interface.
While the invention is particularly directed to the art of 3 G UMTS wireless networks, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications.
Wireless communications networks, also referred to as cellular networks, provide users with mobility while communicating by using a plurality of Base Stations, also referred to as NodeBs, each covering different geographic areas, known as Sectors and/or Cells. The user's Mobile Terminal, also referred to as User Equipment, communicates with one or more NodeBs via RF Links established over Air Interfaces, also referred to as RF Interfaces. The UE transmits information to the NodeBs via Uplinks (UL) and receives information from the NodeBs via Downlinks (DL). Each Base Station is designed to send and receive wireless traffic for multiple Circuit Switched and Packet Switched clients over these RF Links.
3 G UMTS networks are configured with the intention that the RF Link is subject to similar losses in both the UL and the DL. However it is sometimes the case that UL and DL losses are not equal, for instance this situation is commonly seen when a site has incorrect provisioning of external amplifiers or when component faults introduce additional attenuation to one side of the RF path. Such a discrepancy between UL and DL paths is generally described as a “Path Imbalance”, or RF Link Imbalance, or Link Imbalance, and if the imbalance is large enough it can have a serious impact on call performance and stability.
Symptoms of Link Imbalance may include (but are not necessarily limited to: unreliable access or no access to the Network by the mobile terminal, power overshoots, power undershoots, degraded call performance, radio link failures, increased signaling due to link state toggling, and loss of radio link diversity gains.
Although Link Imbalance can have serious consequences it presents some challenges to identify. From the Network side of the air interface, measurements and alarms are not available to highlight this problem without complicated correlation of measurements from the mobile terminal and the Network. Due to this complexity it is not usual to proactively look for errors and, as such, they can go unnoticed for some time. Even if the relevant trace is available, investigation by skilled Network engineers is generally necessary to analyze the results. Needles to say, this procedure is time consuming and potentially inaccurate.
Whenever a call is active the UE can have one or more radio links established between single or multiple nodeB's. When the number of radio links is greater that one, the UE is said to be in Handover (HO). As the UE moves within the Network, radio links between the UE and nodeB(s) will be created and deleted in order to maintain the best combination of links for optimal call performance and stability.
For efficient use of nodeB and UE resources the links in both the UL and DL directions implement a known Power Control Mechanism. The Power Control Mechanism optimizes the amount of power used in each direction by relying on independent feedback mechanisms to continually instruct the other end of the RF Link to increase power, that is “power up”, or decrease power, that is “power down”. If more than one link exists between a UE and the network the UE may have conflicting instructions from different NodeB's to adjust its power. The UE will give priority to any instruction to power down and in this way the minimum required Tx power can be maintained in each direction based on the requirements of the “best” link.
In order to make the decision to add a new link, the Network requires the UE to provide reports based on Quality measurements made for the DL path. Therefore a report indicating that the DL is at a level suitable for adding may instigate a HO procedure. Ideally the UL and DL paths experience similar losses for all nodeB's, so once a HO procedure occurs the relative UL and DL power levels required for all links will be comparable. However if a Link Imbalance exists the UL will be either stronger or weaker in relation to the DL by an amount equal to the imbalance for one of the links.
If the UL is Stronger than expected and the losses in this direction are lower, the nodeB providing this link will instruct the UE to power down until it achieves the optimal Signal to Interference ratio (SiR). This will ultimately lead to the other links in the set experiencing a drop in receive level and consequently SiR. This situation in turn may result in non-optimal performance for the call and failure of the redundant links.
If the UL is Weaker than expected the opposite may occur, that is, the other links in the set will be stronger in the UL and indicate for the UE to power down, while the newly added link may need the power to increase. In this situation the new link will experience poor performance or link failure.
Another consideration when understanding the need to identify RF Link Imbalance is the affect of this issue on 3 G High Speed Downlink Packet Access (HSDPA) calls. For this type of call there is increased impact due to reliance on a single primary link in the set, referred to as the Serving Cell Link. Unlike R99 based calls, HSDPA relies on the use of UL signaling, referred to as HS-DPCCH, directed towards the Serving Cell and this channel has no redundancy across the different radio links between the UE and the Network. Therefore if the primary link degrades or fails, then the HS-DPCCH signaling will also degrade or fail with no possibility of recovering the data from other links in the set. This can result in very poor DL data performance for HSDPA and also the potential for call interruptions depending on the mechanisms implemented for link failure recovery.
The present invention contemplates a new and improved method for detecting RF Link imbalances that resolves the above-referenced difficulties and others.